B cells play a central role in acquired immunity. These cells possess the unique ability to mount a rapid and directed antibody response against foreign antigens, and to act as antigen-presenting cells. To maintain B cell homeostasis and a self-tolerant state, it is important to have a continuous pool of B cell precursors that will mature and migrate to peripheral organs, as well as maintain a process of negative selection to eliminate autoreactive B cells. Dysregulation in the B cell developmental process could lead to a block in B cell development, and thus immune deficiency, or conversely, to an escape and expansion of self-reactive B cells leading to autoimmunity.
Generation of high affinity, somatically hypermutated autoantibodies is one of the hallmarks of autoimmune conditions. The autoantibodies can cause severe tissue damage (e.g., as in lupus nephritis) or loss of blood components (e.g., as in immune thrombocytopenia purpura). The prevailing treatment strategies for autoimmune disorders employ global immunosuppressants that have harmful side effects with long-term use.
Recent discovery of the B cell survival and maturation factor BAFF (also known as TALL-1, THANK, BLyS, zTNF4, and TNFSF13B, and sometimes referred to as neutrokine α, NTN2, Kay, MARCH, TL5, TNFL1, and “63954”) provided a unique opportunity for developing targeted intervention strategies for autoreactive B cell function. Elucidation of the role of BAFF in acquired immunity has been rapid since its first description as a B cell growth factor. BAFF (SEQ ID NO:30) (Accession No. AAD25356) is described in, e.g., Schneider et al (1999) J. Exp. Med., 189:1697-1710; PCT Publication WO 99/12964 and U.S. patent application Ser. No. 09/911,777 (issued as U.S. Pat. No. 6,869,605); and U.S. Pat. Nos. 6,623,941 and 6,689,579. BAFF has been implicated in costimulation of B cells (Moore et al. (1999) Science, 285:260-263; Schneider et al. (1999) J. Exp. Med., 189:1747-1756; Mukhopadhyay et al. (1999) J. Biol. Chem., 274:15978-15981); increased B cell proliferation (Moore et al. (1999) Science, 285:260-263); and increased survival of normally deleted B cells (Khare et al. (2000) Proc. Natl. Acad. Sci., 97:3370-3375; Gross et al. (2000) Nature, 404:995-999; Mackay et al. (1999) J. Exp. Med., 190:1697-1710). Studies have indicated that higher than normal levels of BAFF may contribute to the pathogenesis of autoimmune diseases, such as systemic lupus erythematosus (SLE) and rheumatoid arthritis. For a review, see, e.g., Mackay et al. (2002) Nature Reviews: Immunology, 2:465475; Kalled et al. (2003) Expert Opin. Ther. Targets, 7(1):115-23.
Three cognate receptors for BAFF have been identified: (1) B cell maturation antigen (BCMA; Accession No. S43486; Gross et al. (2000) Nature, 404:995-999; PCT Publication WO 01/12812; U.S. patent application Ser. No. 10/077,137); transmembrane activator and cyclophilin ligand interactor (TACI; Accession No. AAP57629; Gross et al., supra); and more recently, BAFF-R (also called BR3; Accession No. AF373846; Thompson et al. (2001) Science, 293:2108-2111). BAFF-R is the only one of the three receptors that is specific for BAFF (Thompson et al., supra). BCMA and TACI bind not only to BAFF but also to another TNF family ligand, APRIL (Yu et al. (2000) Nat. Immunol., 1:252-256; Wu et al. (2000) J. Biol. Chem., 275:35478-35485; Rennert et al. (2000) J. Exp. Med., 192:1677-1684; PCT Publication WO 01/24811; U.S. patent application Ser. No. 10/115,192).
Soluble forms of BAFF receptors have been made by fusing their extracellular domains to the Fc portion of immunoglobulin. Treatment of normal mice with such a soluble form of TACI or BCMA (TACI-Fc or BCMA-Fc) leads to reduced B cell numbers and a lack of humoral response (Shu et al. (1999) J. Leukoc. Biol., 65:680-683; Yan et al. (2000) Nat. Immunol., 1:37-41; Xia et al. (2000) J. Exp. Med., 192:137-143; Wang et al. (2001) Nat. Immunol., 2:632-637; Yu et al. (2000) Nat. Immunol., 1:252-256). For example, in a mouse model for rheumatoid arthritis, an autoimmune disease that involves both B and T cell components, TACI-Fc substantially inhibits inflammation and slows disease progression (Wang et al. (2001) Nat. Immunol., 2(7):632-637). These effects are thought to be attributed to BAFF sequestration because BAFF-deficient mice have a phenotype similar to that of TACI-Fc- or BCMA-Fc-treated mice (almost complete loss of mature B cells and a severely compromised humoral response) (Schiemann et al. (2001) Science, 293:2111-2114; Gross et al. (2001) Immunity, 15:289-302). More recently, BAFF-specific agents, including BAFF-R-Fc and BAFF antibodies, have been developed for treatment of autoimmune and other disorders (see, e.g., U.S. patent application Ser. Nos. 09/911,777; 10/380,703; 10/045,574; and 60/458,707); Kalled et al. (2003) Expert Opin. Ther. Targets, 7(1):115-23).
Thus, while therapeutic strategies employing BAFF-specific agents already exist, and new drugs targeting the BAFF signaling pathway are being developed, there is a need to provide methods for evaluating and monitoring efficacy of such agents, for selecting optimal responders to such treatments, and for providing improved dosing/timing regimens for those therapeutics.